Brake-booster with complementary hydraulic power-assist

ABSTRACT

Pneumatic brake-booster with additional hydraulic boosting and method for boosting braking using such a booster.  
     A pneumatic-hydraulic hybrid pneumatic brake booster is improved. Additional hydraulic boosting ( 19, 20 ) provided by a hydraulic circuit is provided beyond a saturation point (Psat) in the operation of the pneumatic circuit. Hysteresis-induced instabilities in the pneumatic part of the booster are taken into consideration ( 50 ). A choice is made to compensate (A) for the shortfall in operation of the pneumatic circuit using the hydraulic circuit. In one example, this correction is afforded via a microprocessor which imposes a corrected process signal. Using this signal, pressure shifts in the pneumatic system are neutralized.

[0001] The subject of the present invention is a pneumatic brake-boosterwith additional hydraulic boosting and a method for boosting brakingusing such a booster. An object of the invention is to make theoperation of the additional boost effect more reliable and make abraking action perfectly linear as a function of a braking command. Thefield of application of the invention is more generally that of thegeneral control of the braking of a vehicle.

[0002] A pneumatic brake-booster comprises, in principle, avariable-volume front chamber separated from a rear chamber, the volumeof which is also variable, by a partition formed by a sealed andflexible diaphragm and by a rigid skirt plate. The rigid skirt drives apneumatic piston bearing, via a push rod, on a primary piston of amaster cylinder of a hydraulic braking circuit, typically a tandemmaster cylinder. The front chamber, on the master cylinder side, isconnected pneumatically to a source of vacuum. The rear chamber, on theopposite side to the front chamber, and placed on the brake pedal side,is connected pneumatically, in a way controlled by a valve, to a sourceof driving fluid, typically air at atmospheric pressure. At rest, thatis to say when a driver is not pressing on the brake pedal, the frontand rear chambers are connected together, where as the rear chamber isisolated from atmospheric pressure. Under braking, the front chamber isfirst of all isolated from the rear chamber, then air is let into therear chamber. This letting-in of air has the effect of driving thepartition and of implementing pneumatic boosting of the braking.

[0003] Hydraulic brake boosting effects are known from elsewhere.Typically, an electric motor is connected to a hydraulic pump whichinjects a fluid under pressure into the braking circuits when these arecalled upon. Control of this electric motor is provided by measuring thepressures obtaining in the front and rear chambers of the pneumaticbrake-booster. Use is therefore made of two pressure detectors, whichare connected pneumatically to each of these chambers, so as to measurethe pressure. These detectors provide electrical signals that representthese pressures.

[0004] The additional hydraulic boost system has the main objective, asis known, of contriving to prevent the wheels from locking under suddenbraking. Such a system, which is known as an anti-lock braking system,or as ABS, allows the hydraulic pressure in the braking circuit to bemodulated. This system makes it possible to apply, or alternatively, notto apply, additional hydraulic pressure to the hydraulic circuit so thatthe pressure applied exceeds or does not exceed a limiting pressurebeyond which the wheels will lock up.

[0005] A change to these additional hydraulic boost braking circuits hasbeen dictated by the reduction in weight of the vehicles. What hashappened is that this reduction has led to a reduction in the size ofthe pneumatic brake-booster, the size of the front and rear chambers.Because of the size reduction, and because the high pressure used isatmospheric pressure (which is practically always the same), the boosteffect afforded by the pneumatic boosting has seen a reduction in itseffectiveness. The object of additional hydraulic boosting is thereforeto afford additional boosting, which tends to apply to the hydrauliccircuit a raised hydraulic pressure so that a hydraulic pressure whichis higher than the one needed to cause the wheels of a vehicle beingbraked to lock up can be reached.

[0006] In practice, the pneumatic boost function links the force ofeffort exerted by a driver to the hydraulic pressure of the brakingcircuit, namely the effectiveness of the braking, in a linear way.However, this linear relationship is achieved only as long as the highpressure let into the rear chamber can exert a boosted braking effort.In practice, there is equilibrium in the position of a brake pedal. Thisequilibrium results, on the one hand from the effort exerted by thedriver added to the pneumatic boosting, and, on the other hand, from thehydraulic reaction of the braking circuit. In order to avoid driverfatigue, a ratio between the pneumatic boosting and this driver effort,which is of the order of five, or other values depending on the varioussystems are allowed.

[0007] From the moment when the rear chamber of the pneumatic booster issubjected to atmospheric pressure (and can no longer be subjected to ahigher pressure) and the front chamber is subjected to the maximumdepression that the vacuum pump can produce, the pneumatic boosting nolonger comes into effect. Under these conditions, the additionalcomponent of the braking effort is provided only by the driver. Thepressure obtaining in the hydraulic circuit when this phenomenon occursis known as the saturation pressure.

[0008] The curve of the correspondence between the pressure in themaster cylinder and the force exerted by the driver thereforeexperiences an initial increase along a slope known as the boost slope,which is fairly steep as far as this saturation pressure value. It thenevolves with a far shallower slope, due only to the effort on the partof the driver.

[0009] When the pneumatic boost braking system was bulky, thissaturation pressure was above the pressure at which the wheels of thevehicle locked up. It was then left to the driver, or alternatively toan anti-lock braking system, to keep this other problem in check.However, because of the reduction in the size of pneumatic boosters, thesaturation pressure is now reached in the hydraulic circuit before thepressure at which the wheels lock up is reached. To this end, anadditional hydraulic boost circuit, typically a hydraulic pump, takesover from this pneumatic boosting action.

[0010] This additional boosting, for good driveability, has nonthelessto occur in continuity with the efforts deployed by this driver. Thismeans that the ratio between the effort exerted by the driver and thehydraulic pressure acting on the wheels have to be the same as, orsimilar to, the ratio that already existed at the time that theadditional hydraulic boosting comes into operation. Corresponding to thesaturation pressure in the hydraulic circuit, there is an effort knownas the saturation effort for which this saturation pressure is reached.The effort exerted by the driver is therefore measured in the additionalhydraulic boost circuits, the saturation effort is subtracted, and thedifference is multiplied by the coefficient of amplification thatalready existed during pneumatic boosting.

[0011] By taking this approach the result obtained as far as the driveris concerned is therefore that the boosting occurs always with the sameeffectiveness, whether it is pneumatic or hydraulic in origin. Thedriver is unaware of the difference. In order to measure the saturationpressure or the saturation effort, there are various conceivablesystems, the principle of which is to compare with one another thepressures that exist at the time of this saturation in the frontchambers, rear chambers or various points of the hydraulic circuit.

[0012] However, under certain circumstances such an additional hydraulicboost system operates in an abnormal and troublesome way. This abnormaland troublesome operation stems from a sharp increase in the vacuum inthe front chamber during braking. This sharp increase in the vacuum may,for example, be brought about by engaging a lower gear in the gearbox.This lower gear itself leads to the engine turning over faster (enginebraking) which leads to a greater intake and therefore a strongerdepression created in the front chamber.

[0013] The origin of this greater depression in the front chamber mayalso be the result of operation of a non-return valve present on theintake of the vacuum into this front chamber. What happens is that acalibrated leakage of low value, for example 15 millibar per second,causes an increase in pressure in the front chamber. It is possible forthe pressure in the front chamber, as a result of this leakage, toexceed the set pressure of this non-return valve. Under theseconditions, the front chamber is once again subjected to the vacuum. Theset pressure of a non-return valve such as this is of the order of 25millibar. As a result, a sudden depression of the order of 25 millibarmay be applied to the front chamber.

[0014] If both phenomena, the engaging of a lower gear in the gearbox,and the refreshing of the vacuum in the front chamber, occursimultaneously, it is possible for the front chamber to experience anadditional depression, of the order of 50 millibar. In this case, and ifthe pneumatic booster is in the saturation region, misadjustment of theadditional hydraulic boost system occurs. Everything therefore happensas if this sharply applied depression were interpreted by the additionalhydraulic boost circuit as a sudden effort applied by the driver. Theamplification phenomenon afforded by the additional hydraulic boostingthen leads to excessive braking which tends to cause the wheels to lockup immediately. In practice, this phenomenon is compensated for by theABS cutting in, so that no decremental effect on driving occurs.However, in the exceptional case where such a situation might occur, asolution such as this is disagreeable and unsatisfactory.

[0015] The invention seeks to overcome this problem. In the invention,the solution found consists in compensating for the offset, thehysteresis-induced operation of the pneumatic boost circuit by rapidrecalibration in the hydraulic circuit. In order to determine thishysteresis-type operation, the invention therefore measures parametersconcerned with pressures obtaining in the front and/or rear chambersand/or in the hydraulic circuit prior to the implementation of theadditional hydraulic boosting. Then, constantly, when this additionalhydraulic boosting is activated, these parameters with their currentvalues are compared with the parameters already recorded. If, throughthese comparisons, a modification to the conditions in which theadditional hydraulic boosting has been implemented is detected, thisadditional hydraulic boosting is modified to avoid untimelymisadjustments: overbraking or even brake release. In practice, in theinvention, the pressure is measured in the front chamber prior toimplementation of the additional hydraulic boost circuit, and thispressure in the front chamber is measured again constantly at the timeof use. The value of the saturation pressure that is used is modified asa function of this variation (if any) of this depression in the frontchamber. By taking this approach, jerky braking is avoided.

[0016] A subject of the invention is therefore a pneumatic brake-boostercomprising a front chamber that can be connected to a source of vacuum,a rear chamber that can be connected to a high-pressure inlet, a sealedmoving partition between the two chambers, a moving gear carried alongwith the moving partition and connected to a hydraulic braking circuit,a device for letting a high-pressure fluid into the rear chamber at thetime of braking, and additional hydraulic boost means equipped with ahydraulic actuator, characterized in that these additional boost meansfurther comprise an electronic circuit

[0017] which stores in memory a first pressure obtaining in one of thechambers, prior to implementation of these additional boost means,

[0018] which measures a second pressure obtaining in one of the chambersduring this implementation, and

[0019] which controls the actuator as a function of a difference betweenthis first and this second pressure.

[0020] In a preferred embodiment, the measurements are taken in thefront chamber.

[0021] Another subject of the invention is a method for boosting brakingusing a pneumatic brake-booster in which

[0022] a pneumatic brake boosting device is implemented in the booster,

[0023] a hydraulic brake boosting device is implemented in addition inthe booster,

[0024] a function of the boosting by the hydraulic device depends on afirst depression state in the pneumatic device prior to implementationof the hydraulic device, characterized in that, during braking,

[0025] a second depression state is measured in the pneumatic deviceafter implementation of the hydraulic device, and

[0026] the function of the hydraulic boosting is modified as a functionof a difference in measurements between this first and this secondstate.

[0027] The invention will be better understood upon reading thedescription which follows and upon examining the accompanying figures.These are given merely by way of non-limiting indication of theinvention. The figures show:

[0028]FIG. 1: A schematic depiction of a pneumatic booster for boostingthe hydraulic braking according to the invention;

[0029]FIG. 2: A diagram showing the correspondence between an effortexerted by a driver and a pressure obtaining in a brake master cylinder,with and without the additional hydraulic boosting device of theinvention.

[0030]FIG. 1 shows a pneumatic brake-booster according to the inventioncomprising a front chamber 1 that can be connected to a source of vacuum2. Typically, the source 2 may consist of inlet gases being trapped fora vehicle with a petrol engine. In the case of a vehicle with a dieselengine, use would be made of an external vacuum pump. The pneumaticbrake-booster also comprises a rear chamber 3 that can be connected, forexample via a schematic valve 4, to a high-pressure inlet 5 (typicallyambient air at atmospheric pressure Atm). The pneumatic brake-boosteralso comprises a moving partition 6 habitually equipped with a rigidskirt and with a sealed diaphragm. The diaphragm prevents pneumaticcommunication between the two chambers. The diaphragm 6 is pierced witha sealed orifice 7 to allow a moving gear 8 to pass. The gear 8 ismechanically connected on the one hand to a brake pedal 9 and, on theother hand, to a hydraulic braking circuit 10. The principle of boostingafforded by such a pneumatic brake-booster is as follows. Under theaction of the pedal 9, the moving gear 8 plunges into the rear chamberuncovering the valve 4 via which ambient air is let into the rearchamber 3. The ambient air then exerts pressure on the partition 6which, via a thrust face 11 secured to the moving gear 8, drives themoving gear 8 in such a way that one end 12 thereof actuates thehydraulic braking circuit 10.

[0031] In addition, in the preferred example, the pneumaticbrake-booster comprises a helical flexible hose 13. The helical flexiblehose 13 allows the rear chamber 3 to be connected in a sealed manner toa pressure detector 14 mounted at the front of the front chamber. Thehelical flexible hose 13 opens into the rear chamber 3, through thepartition 6, via a pipe 15. Another pipe 16 connects the flexible hose13 to the detector 14. The detector 14 also faces another orifice of thefront chamber. The detector 14 is thus capable of producing two signalstransmitted by connections 17 and 18 to an electronic control circuit19. The detector 14 therefore picks up pressure information transmittedby the connection 17 which relates to a pressure Pfc obtaining in thefront chamber 1 and, by the connection 18, to a pressure Prc obtainingin the rear chamber 3.

[0032] Means for the additional hydraulic boosting of the pneumaticbrake-booster comprise, in principle, a hydraulic actuator 20 hereequipped with an electric motor 21 which drives a pump 22. Via acoupling 23, the pump 22 injects a hydraulic fluid into one of the ducts24 of the hydraulic circuit 10. Implementation of the hydraulic boosting20 supplements (or neutralizes in the case of the anti-lock brakingsystem) the rise in pressure in the duct 24.

[0033] In the invention, the additional hydraulic boost means furthercomprise the electronic circuit 19, here depicted in a conventional wayby a microprocessor 25 connected by a control, address and data bus 26to a program memory 27, to a data memory 28, to an input interface 29and to an output interface 30. The connections 17 and 18 are connectedto the interface 29 together with a connection 31 from a detector 32 ofthe pressure Pc obtaining in a master cylinder 33 of the hydrauliccircuit 10. In practice, the electronic circuit 19 may form amicrocontroller (microprocessor equipped with its program memory on oneand the same integrated circuit).

[0034] The program memory 27 comprises, in a known way, a first program34 intended to produce a control signal A to be applied, by theinterface 30, to control the hydraulic boosting 20. The program memory27 also comprises, according to the invention, another program 35, thecontents of which will be explained later on. From a practical point ofview, the programs 34 and 35 may be merged into one single program, itbeing possible for the electronic circuit 19 to be considered as being astate machine with programmed transitions.

[0035]FIG. 2 shows the principle of brake boosting for a system equippedwith pneumatic then hydraulic boosting. On the abscissa axis, thediagram indicates the efforts F exerted by the driver of the vehicle tobrake his vehicle. On the ordinate axis are depicted the pressures Pc inthe master cylinder which result from the efforts F. When the driverbrakes, to begin with, his effort opposes the reactions of the pedalalone. Then, for an effort higher than a given threshold Fs, the boostedbraking comes into operation. At the time of this boosted braking, thecurve showing the correspondence between the pressure Pc and the effortF approximately follows a straight line 36 known as the boost line,indicating that the pressure in the master cylinder is the result, onthe one hand, of the effort F and, on the other hand, of the boostingafforded by the pneumatic brake-booster 1.

[0036] This proportional relationship is applied up to the point wherethe force F reaches a force known as the saturation force Fsat. Apressure in the master cylinder equal to Psat corresponds to the forceFsat. Where Pc is equal to Psat, the pressure in the rear chamber 3 hasbecome equal to atmospheric pressure, and additional opening of thevalve 4 can no longer provide pneumatic boosting. In this case, withoutadditional boosting, the correspondence curve follows a straight line 37of direct correspondence in which only the effort applied by the drivercauses the pressure in the hydraulic circuit 10 to increase.

[0037]FIG. 2 also shows a horizontal line 38 indicating the pressure Pbfor which the wheels of the vehicle lock up. In the exampleschematically shown, because of the reduction in the size of thepneumatic brake-booster, the pneumatic boosting is not enough to achievethis lock up pressure without exaggerated pressure from the foot on thepedal 9. FIG. 2 also shows, schematically by the arrow 39, the effect ofthe hydraulic amplification afforded by the program 34 of the circuit19. In principle, this program 34 measures the difference in pressurebetween the current pressure Pc in the master cylinder and the pressurePsat at which boosting according to the straight line 36 ended. Thepressure difference Pc−Psat is multiplied by a multiplicativecoefficient by the program 34, and a corresponding command A is appliedto the input of the motor 21 so that the pressure available in thecircuit 24 is increased, upwards of the current pressure Pc, by asupplement which is proportional to the difference between the pressuresPc and Psat. This is represented schematically by the continuation 40 ofthe boost line 36.

[0038] The problem caused by a sharp increase in the depression in thefront chamber 1 is depicted schematically by the curve 41 in dotted linewhich is roughly parallel to the straight line 37 and situated slightlyabove it. When the pressure in the front chamber drops sharply, as aresult of this depression, and because the foot of the driver is not atthat moment moving on the pedal (the phenomenon is too swift), the skirt6 is moved until the above-mentioned equilibrium is regained. As, on theone side, the combination of the effort by the foot plus the depressionincreases sharply, this then results in a sharp increase in the currentpressure Pc measured by the detector 32. Instead then of using, as acorrective term, a difference current pressure Pc (before the suddendepression)−Psat, the difference taken into consideration is now equalto the previous difference increased by the sudden overpressure. Whatthis means is that instead of the correspondence 39, the additionalhydraulic boosting provides a boost effect 42. As this boost effect 42is too great, the effort exerted on the wheel exceeds the lock uppressure Pb. In this case, the wheel locks up, the anti-lock brakingsystem comes into operation. This results in an unpleasant drivingexperience.

[0039] In the invention, in order to overcome this problem, a decisionis taken during the second phase 43 of the program 35 to check, in realtime, one or more pressures at various parts of the circuit bycomparison with the values that these pressures had at the time when thepneumatic boosting reached saturation. The phase 43 is after a phase 44of this same program 35. This phase 44 will be explained later on. Thesecond phase 43 comprising a first test 45 during which measurements aretaken to determine whether the pressure in the rear chamber is lowerthan atmospheric pressure: Prc lower than Patm. What happens is that ifthis pressure Prc is lower than atmospheric pressure, the pneumaticbooster 1 still has the capability to afford its assistance. If this isthe case, the test 45 loops back on itself at high frequency, forexample of 1 MHz. The entire microprocessor 25 may run at a higher speed(easily 100 MHz at the present time). By taking this approach it ispossible to detect, to within a microsecond, the exact moment at whichthe pressure in the rear chamber 3 reaches atmospheric pressure. If needbe, the clock frequencies of the microprocessor 25 will be changed inorder to take temporal precision considerations into account.

[0040] When the pressure in the rear chamber is no longer lower thanatmospheric pressure, during step 46 initiated immediately after thistest 45 (in the next microsecond), the current pressure Pc is measuredand is stored in memory as value Psat in a recording zone 47 of thememory 28. The pressure Pfc in the front chamber is also measured andstored in memory in zone 48 under the name of a variable Pmen. Once thishas been done, two parameters which are important in monitoringphenomena 49 of increase in depression in the front chamber 1 have beenmeasured. However, it could be shown that detecting these phenomena 49can be achieved by measuring pressures at other points. Indeed, all thepressure values are linked to one another. A detection delay maypossibly be observed if other pressures are measured for this purpose.

[0041] During a step 50 immediately following step 46, for example inthe same microsecond as step 46, a variable known as Pestim is made toadopt a value equal to Psat+α(Pfc−Pmem). In doing this, when Pfc isequal to Pmem, the result obtained is that the first time Pestim isequal to Psat. On the other hand, it can be clearly seen that if thecurrent pressure in the front chamber Pfc, is caused to change, thecorrection afforded by the step 50 makes it possible to take thedifference into account. To do this, in the program 34, the calculationof the command A is performed on the basis of Pestim. Any difference cantherefore be tolerated without difficulty, because they are neutralizedby step 50.

[0042] It then remains to be determined, on the one hand, how the sharpincrease in the depression in the front chamber is detected, and how,incidentally, the coefficient α is calculated. To detect the sharpdepression in the front chamber 1, a test 51 (after step 50) allows thecurrent pressure Pc to be compared with the corrected value of thesaturation pressure: Pestim. What happens is that under normalcircumstances the current pressure Pc is greater than Pestim (which atthe start is equal to Psat). If this is the case, the step 50 and thetest 51 are undertaken in a loop. In step 50, each time a new value ofPfc is taken into consideration.

[0043] The mere presence of step 50 and of the normal looping back ofthe test 51 when this pressure Pc is greater than Pestim leads, at afrequency of recurrence equal to the recurrence of this looping-back, tothe modification Pfc in the depression in the front chamber being takenimmediately into consideration. For example, the loop-back frequency ofstep 50 and the test 51 can be allowed to be equal to the order of 1MHz. This looping-back leads to a slight delay in perception equal toone microsecond. However, given the inertia of the various mechanismsinvolved, the motor 21, the pump 22, not forgetting the braking linkagesthemselves, the correction is applied before the error can havedetrimental effects.

[0044] On the other hand, if the current pressure Pc drops below theestimated pressure Pestim, that is to say, in practice, below Psat, thenthe additional hydraulic boosting is no longer needed. In this case, thesystem loops back to the first phase 44.

[0045] In practice, during that phase, the value of the coefficient α ismeasured. The value of the coefficient α is quite simply arepresentation of the slope of the boost line 36. Given the existence ofthe triggering threshold Fs, a decision is taken during the test 52 inphase 44, not to begin to take the slope 36 into consideration unlessthe current pressure Pc measured by the sensor 32 is higher than a lowerlearning pressure Pl, itself located after the jump of Fs. What happensin the invention is that it is reckoned that if this current pressure ishigher than this lower learning pressure, the correspondence of thecurve 36 is linear and can be taken into consideration. In this case,during a later step 53, α is calculated. The value of α is a function,at any moment, of the pressure in the front chamber Pfc and in the rearchamber Prc. Moreover, this correspondence could be established also byusing the measured current pressure Pc. Indeed, given the aforementionedequilibrium, these three pressures are mutually coherent and connectedby a unique relationship. In so doing, the calculation of step 53 isrepeated as long as a subsequent test 54 has not revealed that thecurrent hydraulic pressure has reached a higher learning threshold Ph.During this period, the test 52 and the step 53 are performed in a loop.The calculation of step 53 corresponds to the adjustments which wereundertaken in the prior art in order to ensure continuity of theproportions 36 and 40. The specific feature of the invention, however,lies in the fact that this calculation is undertaken upon each freshbraking action. As a result, no standardized calibration of theequipment is needed: this calibration is performed automatically. Ittherefore takes account of the ageing of the device.

[0046] When this threshold Ph is reached, the program 35 enters thephase 43. It is not necessary for the looping back of the steps 52, 53and 54 to be as swift as the looping back of the test 45 and of steps 50and 51. However, for reasons of consistency, it may be permissible forthe scrutinization to occur at the same frequency, 1 MHz in one example.In other words, the first phase 44 is intended to measure α in a part ofthe curve 36 where the curve is perfectly linear. Given the offsets atthe origin of the curve 36, it may be necessary to formulate thecoefficient α and the slope of this curve on the basis of two successivemeasurements (between an end point and an origin point of a segmentcarried by the straight line 36). If the intention is to simplify thecalculation, this straight line may be assumed to pass approximatelythrough the origin of the x- and y-axes, the value of the slope α beingobtained using just a single point, the last one measured before thetest 54.

1. Pneumatic brake-booster comprising a front chamber (1) that can beconnected to a source of vacuum (2), a rear chamber (3) that can beconnected to a high-pressure inlet (4), a sealed moving partition (6)between the two chambers, a moving gear (8) carried along with themoving partition and connected to a hydraulic braking circuit (10), adevice (5) for letting a high-pressure fluid into the rear chamber atthe time of braking, and additional hydraulic boost means (19, 20)equipped with a hydraulic actuator, characterized in that theseadditional boost means further comprise an electronic circuit (19) whichstores in memory (48) a first pressure (Pmem) obtaining in one (1) ofthe chambers, prior to implementation of these additional boost means,which measures (14, 17) a second pressure (Pfc) obtaining in one (1) ofthe chambers during this implementation, and which controls (34, A) theactuator as a function (a) of a difference (Pestim) between this firstand this second pressure.
 2. Booster according to claim 1, characterizedin that the electronic circuit comprises a microprocessor (25) and aprogrammable memory (27) containing a recorded program (34, 35). 3.Booster according to claim 2, characterized in that the recorded programcomprises steps for producing a signal (A) to be applied to thehydraulic actuator, the value of which is a function of the differencebetween a third pressure measurement (Pc) of the hydraulic pressure at apoint in the hydraulic circuit and an estimated value of this hydraulicpressure.
 4. Booster according to claim 3, characterized in that theestimated value (Pestim) of the hydraulic pressure is compared (51) witha current value (PC) of this hydraulic pressure, and this estimatedpressure is corrected (50) when this estimated pressure is lower thanthis current pressure.
 5. Booster according to any of claims 1 to 4,characterized in that one of the chambers in which the first and secondpressures are measured is the front chamber (1).
 6. Method for boostingbraking using a pneumatic brake-booster in which a pneumatic brakeboosting device (1-8) is implemented in the booster, a hydraulic brakeboosting device (19, 20) is implemented in addition (23) in the booster,a function of the boosting by the hydraulic device depends on a firstdepression state (Pmem) in the pneumatic device prior to implementationof the hydraulic device, characterized in that, during braking, a seconddepression state (Pfc) is measured (14) in the pneumatic device afterimplementation of the hydraulic device, and the function of thehydraulic boosting is modified (50) as a function of a difference inmeasurements between this first and this second state.
 7. Methodaccording to claim 6, characterized in that the function of thehydraulic boosting is established during a first phase (44) of brakingprior to implementation of the hydraulic device.
 8. Method according toany of claims 6 to 7, characterized in that a signal (A) for controllinghydraulic boosting and the value of which is proportional to thedifference between a third pressure measurement (Pc) of the currenthydraulic pressure at a point in the hydraulic boost circuit and anestimated value (Pestim) of this hydraulic pressure is produced (34). 9.Method according to claim 8, characterized in that the estimated valueof the hydraulic pressure is compared with a current value of thishydraulic pressure, and this estimated pressure is corrected as long asthis estimated pressure remains lower than this current pressure.